1. Field of the Invention
The present invention relates to an angled chisel used for cutting a bone of a lumbar spine in minimally invasive surgery for lumbar spinal canal stenosis.
2. Description of the Related Art
FIG. 1 is an axial cross-sectional view illustrating a normal lumbar spinal canal. A lumbar spinal canal 1 is a lumen surrounded by a facet joint 2, a ligamentum flavum 3, a vertebral arch 4, an intervertebral disc, and the like. A spinal cord, a dura mater, and nerve roots run in the lumbar spinal canal 1. The lumbar spinal canal stenosis is a disease which causes neurological symptoms in legs by narrowing of the spinal canal due to hypertrophy of the facet joint, hyperplasy of the ligamentum flavum, or the like as illustrated in FIG. 2.
Posterior decompression which is common as a surgery treatment for the lumbar spinal canal stenosis is a therapeutic approach in which bones of the vertebral arch and the facet joint are partially excised and the ligamentum flavum is also excised so as to expand the spinal canal. As such posterior decompression, an operative procedure which is recently spread as the minimally invasive surgery is a micro endoscopic discectomy (MED). The MED is an operative procedure in which a tubular retractor having a diameter of slightly smaller than 2 cm is installed at the backside of the vertebral arch and the surgery is completed in the tubular retractor under an endoscope. A spinous process and a supraspinous ligament are important as lumbar spine posterior stabilizing mechanisms. With the MED, these tissues can be conserved and invasion of muscles around these tissues can be minimized.
In the MED, an operation is performed in an inner side of a tubular retractor 5 which is an elongated tube as illustrated in FIG. 2. Therefore, a surgical region is limited by an angle and a position of the tubular retractor. An upper portion of the tubular retractor cannot be inclined to the side of a spinous process 6 due to obstruction of the spinous process 6 at the approach side. Therefore, there has been a problem that an angle at which a medial facet is cut at the approach side is as indicated by a solid line 7 and decompression at the approach side cannot be sufficiently made unless extensive bone is excised. Further, if too much bone is excised, many portions of the facet joint are removed or facet fracture is caused. This causes a risk that instability is increased.
In order to conserve the facet bone as indicated by bone-cutting lines of dashed lines 8 as much as possible, it is ideal that a bone is cut outward so as to excise the medial facet at the approach side more selectively. In order to achieve such an object, a chisel of which blade is bent with respect to a handle is already present. However, as a user hammers the chisel so as to advance the chisel, the chisel does not advance in the direction of the bending blade and slides in many cases.
This is because a point 10 to be hammered is located not on an extended line 12 in the direction opposite to the direction of the blade 11 but on an end of a handle 9 for gripping on the chisel as illustrated in FIG. 3. If the chisel is hammered in a state where a tip of the blade thereof is not inserted into a bone, the chisel advances on a line 13 connecting the tip of the chisel and the point 10. That is, in such a case, the chisel does not advance in the direction of the blade and slides on a surface of the bone.